Oral vaccines

ABSTRACT

This invention features a composition that includes a multiple-cell organism for use as food for an aquatic animal (e.g., a fish or a shrimp), and a single-cell organism fed to, and as a result, bioencapsulated by, the multiple-cell organism. The single-cell organism has been transformed to express a recombinant antigen that induces an immune response in the aquatic animal.

RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority to U.S. application Ser. No. 09/946,273, filed Sep. 5, 2001, now U.S. Pat. No. 6,872,386 issued Mar. 29, 2005, the contents of which are incorporated herein by reference.

BACKGROUND

There is a need for introducing a foreign protein in an aquatic animal. Such a foreign protein may lead to an efficacious immunization of aquatic animals. For example, infectious diseases are common on fish farms, due to intensive fish farming that facilitates the transmission of pathogens in an aqueous environment. Dunn et al. (1990) Aquaculture Engineering 9: 23. Preventing aquatic animal diseases by oral vaccination has several advantages over other methods: It is non-stressful, requires little labor, and can be applied at a large scale. However, many oral vaccines have been found ineffective as a result of failure to uptake sufficient dosage of antigen, poor antigen delivery and antigen degradation in the digestive tract.

SUMMARY

This invention relates to a novel composition, e.g., an oral vaccine useful for immunizing an aquatic animal (e.g., a fish or a shrimp), against an infectious disease (e.g., a bacterial, viral, or parasitic disease).

In one aspect, the present invention features a composition, e.g., an oral vaccine that includes a multiple-cell organism for use as food for an aquatic animal to be vaccinated, and a single-cell organism fed to, and as a result, bioencapsulated by, the multiple-cell organism. The single-cell organism has been transformed to express a recombinant antigen that can induce an immune response in the aquatic animal and thereby vaccinate the aquatic animal. The term “food” as used herein includes food (e.g., starting feed for larvae, feed additive, and artificial feed) itself and a food additive. More specifically, the single-cell organism contains a heterologous nucleic acid encoding an amino acid sequence of an antigen of interest. The nucleic acid is in a recombinant vector, which also includes one or more regulatory sequences (e.g., promoters or enhancers) operatively linked to the nucleic acid to be expressed. The antigen need not be the wild-type amino acid sequence found in a naturally occurring gene, as long as it is capable of inducing an immune response. For example, a fish oral vaccine includes artemia (the multiple-cell organism) that has fed on, and encapsulates Escherichia coli (the single cell organism), which has been transformed to express a bacterial or a viral antigen, e.g., nervous necrosis virus envelope protein, a Photobacterium protein, or an immunogenic fragment thereof.

In another aspect, the present invention features a method for preparing a composition, such as an oral vaccine. The method includes (1) providing a single-cell organism which has been transformed to express a recombinant antigen that can induce an immune response, either adaptive or innate immune response, in an aquatic animal and thereby vaccinate the aquatic animal, and (2) feeding the single-cell organism to a multiple-cell organism. As a result of the feeding, the multiple-cell organism (e.g., artemia, rotifer, algae, a paramecium, or an oyster embryo) bioencapsulates the single-cell organism (e.g., a bacterium or yeast). Such a multiple-cell organism can be fed to an aquatic animal as an oral vaccine.

Also within the scope of this invention is a method for orally delivering a composition, e.g., a vaccine to an aquatic animal by feeding a composition described above to the animal.

Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAILED DESCRIPTION

The present invention relates to a composition for delivering a foreign polypeptide to an aquatic animal. The foreign polypeptide can be any of interest. For example, it can be a therapeutic polypeptide, such as a vaccine, an immune stimulator, an antibody chain, a biologically active peptide, a cytokine or its receptors, a growth factor or its receptor, and an enzyme. One can obtain nucleotide sequences encoding these polypeptides from various public databases, e.g., GenBank and the Kabat database (Ramirez-Benitez Mdel et al., Biosystems. 2001 61:125-131).

In one embodiment, the composition is an oral vaccine for immunization of an aquatic animal against infectious diseases, either for prophylactic vaccination or for therapeutic vaccination. The oral vaccine includes a multiple-cell organism that has fed on, and therefore contains, a single-cell organism. The single-cell organism fed to the multiple-cell organism has been transformed to express a recombinant antigen that can induce an immune response in the aquatic animal. The just-described multiple-cell organism, fed to the aquatic animal, serves as an oral vaccine to the animal. In other words, an antigen, being expressed in a single-cell, multiple-cell organism or in a aquatic animal, is delivered to the aquatic animal via two steps of feeding, i.e., the above-described single-cell organism fed to the multiple-cell organism, and the multiple-cell organism from previous step fed to the aquatic animal. As a result of the delivery, the antigen can induce an immune response in the animal.

What antigen to be expressed of course depends on whether the induced immune response is against a targeted pathogen. Having the identified antigen that triggers the immune response, one can clone it into a recombinant vector that includes a nucleic acid encoding the antigen and one or more regulatory sequences operatively linked to the nucleic acid. The regulatory sequences can be those that direct constitutive expression of the antigen, as well as inducible sequences. The recombinant vector can be designed based on such factors as the single-cell organism to be transformed by it. It may contain more than one nucleic acid encoding different antigens. For example, a recombinant vector contains nucleic acids encoding two antigens, which can induce immune responses against the same or different pathogens. Alternatively, the recombinant vector may contain a nucleic acid encoding a polypeptide (e.g., a helper epitope) that is not antigenic, but itself or its encoded peptide serves to enhance an immune response against a targeted pathogen.

The just-described recombinant vector is introduced into a suitable single-cell organism via conventional transformation or transfection techniques, including a variety of art-recognized techniques for introducing a foreign nucleic acid (e.g., DNA) into a suitable host single-cell organism, e.g., calcium phosphate or calcium chloride co-precipitation, DEAE-dextran-mediated transfection, lipofection, or electroporation. Examples of suitable single-cell organisms are described, for example, in Goeddel (1990) Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. One or more just-described single-cell organisms are then fed to a suitable multiple-cell organism, a food or food additive to an aquatic animal to be vaccinated. Accordingly, the just-obtained multiple-cell organism serves as an oral vaccine, in which an antigen is in the single-cell organism that is bioencapsulated by the multiple-cell organism. Unexpectedly, such an oral vaccine has been found effective due to difficult antigen degradation in the digestive tract of the aquatic animal.

The specific example below is to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extent. All publications recited herein are hereby incorporated by reference in their entirety.

Example 1 Materials and Methods

Animals. Zebra fish, Danio rerio, were used. Zebra fish were kept at a suitable temperature such as 28.5° C., and reared in re-circulated, filtered water. At the start of each experiment, fish were 50 days old. Until vaccination, fish were fed with regular brine shrimp (artemia nauplii). After vaccination, feeding with pelleted dry food commenced amounting to 5% of fish body weight per day.

Plasmid construction and antigen preparation. Pseudomonas aeruginosa (PE toxin) is a bacterium that causes nosocomial infections, and has a LD₅₀ value (lethal dose fifty) of 1 μg for a mouse. Also recombinant Pseudomonas exotoxin A (PE) can elicit effective protection against native PE toxin challenge in an animal model.

Plasmid pJH4, which encodes a PE gene without a signal peptide and a c-terminal terminus, was a gift from Dr. Hwang (Hwang et al. (1987) Cell. 48: 129-136). The PE gene was isolated from pJH4 by digestion with EcoR I and Hind III, and the isolated PE gene was inserted into another plasmid pET24a with the same restriction enzyme cohesive ends (EcoR I and Hind III), thereby producing the plasmid pET24a-PE. The resulting plasmid was transformed into E. coli strain BL21(DE3). The bacteria containing the plasmid were cultured at 37° C. in LB broth with kanamycine at a concentration of 50 μg/mL. When absorbance at 600 nm reached 0.6, isopropyl-1-thio-β-D-galactoside was added to a final concentration of 1 mM. After 150 min, the bacteria were harvested, washed twice with phosphate-buffered saline (PBS), and thus, PE-enriched E. coli was obtained. The bacteria were kept at −20° C. until fed to artemia, and the expressed PE protein was analyzed by using SDS-PAGE.

PE-enriched artemia nauplii preparation. Artemia nauplii were hatched in suitable hatching condition, such as in fresh seawater at 29±1° C. or in a hatching solution (sea water:fresh water/2:1) at 25° C., under continuous light and sufficient air. More specifically, four grams of Artemia eggs (INVE Artemia cysts, Belgium) were suspended in fresh water for 30 minute. They were then transferred to 5 liters of the just-described hatching solution and incubate at 25° C. with gentle aeration for up to 24 hours until Artemia larva hatched. Forty hours after hatch, artemia were collected and fed with PE-enriched E. coli (or E. coli control). The larva were resuspended in seawater to a density of 500 larvae/ml and cultured with aeration until they were ready to eat bacteria. To feed them with bacteria, a concentrated stock solution of desired bacteria was added to the Artemia culture to a concentration of 108 bacteria/ml. The resultant mixture was further cultured with gentle aeration at 25° C. for 120 minutes. After feeding, artemia nauplii were collected, and washed five times with PBS. Artemia nauplii were fed to zebra fish immediately, or kept at −20° C. for subsequent use or analysis.

SDS-PAGE electrophoresis and western blotting. Total protein from an artemia sample was quantified by Protein Assay Kit (BIO-RAD, 2000 Alfred Nobel Drive, Hercules, Calif.) and PE protein in an artemia sample was analyzed by using SDS-PAGE and western blot. The sample was homogenized in Laemmli buffer (Laemmli, U. K. (1970) Nature (London) 227: 650-685), and boiled for 5 minutes. The PE protein was analyzed by 8% polyacrylamide gels containing SDS (SDS-PAGE), and stained with Coomassie blue after electrophoresis. It was more than 40% in the total E. coli-expressed proteins. In an immunoblotting assay, the PE protein in the sample was electrophoresed on gels, transferred to a nitrocellulose paper; incubated with the rabbit anti-PE antibody and a second antibody (goat anti-rabbit conjugated with alkaline phosphatase), and then stained with an alkaline phosphatase substrate (BIO-RAD). Procedures were carried out under conditions recommended by the supplier (BIO-RAD). The anti-PE antibody was generated in rabbits by immunization. 250 μg of the PE protein emulsified in Freund's complete adjuvant was administered to the rabbits. The pooled rabbit-anti-PE antibody fractions were prepared for western blotting analysis.

Vaccination schedule. PE-enriched artemia nauplii were fed to zebra fish that have been starved for 24 hours before vaccination. During feeding, the water flow was stopped. Fish were fed four times a day. Control fish (non-vaccinated fish) were fed with control artemia nauplii. After 6 weeks, both vaccinated and non-vaccinated fish were fed with pelleted dry food. After 3 weeks, fish were challenged with a PE toxin (Sigma)

Challenge test with PE toxin. In order to determine LD₅₀ values of PE in zebra fish, five experimental groups (six non-vaccinated fish each) were tested. Five groups were starved for 24 hours, anaesthetized by using 200 ppm of 2-phenoxyethanol, and intraperitonally injected with 0, 0.5, 1, 1.5 and 2 μg of the PE toxin, respectively. A LD₅₀ value of PE for zebra fish was thus determined at the range of 1-1.2 μg in 50 days old zebra fish. In a subsequent experiment, vaccinated fish were challenged with 1.2 μg of the PE toxin.

Immunohistochemistry study of uptake of PE in fish. After feeding with the enriched artemia nauplii, six groups (three fish each) were killed at the time 0, 1, 2, 3, 6, and 12 hours, respectively, and put on ice for 1 minute. Fish was dissected, fixed in 4% formalin overnight, and after rinsing with PBS and dehydration, embedded in paraffin wax. Continuous sections were cut, mounted on poly-L-lysine treated slides, and incubated with a 3% H₂O₂-containing PBS solution to inactivate endogenous peroxidase. The slides were then rinsed with PBS-E (PBS with 1 mM EDTA and dehydrated with 50% (v/v, 50% ethanol and 50% water) and 90% ethanol (v/v, 90% ethanol and 10% water). The PE protein was detected by an immunoperoxidase reaction using rabbit-anti-PE serum (1:1000) and a goat-anti-rabbit horseradish peroxidase (HRP) conjugate (1:200). Conjugates were visualized with a HRP substrate (0.012% v/v hydrogen peroxide in water) and chromogen (0.4 mg/mL). After aminoethylcarbazole was applied to the sections for 15 minutes, the slides were washed twice with PBS-E. The sections was counterstained with acid hematoxylin for 5 minutes, blued with aqueous ammonia for 1 min, and then rinsed with PBS. Detection was carried out by using water as mounting medium, and slides were viewed under a microscope at a magnification of 40-400×. Tissues containing the recombinant PE were stained as reddish-brown color.

Results

Determination of LD₅₀. Since the weight of zebra fish varies even at same age, the LD₅₀ value of the PE toxin for 50 days old zebra fish was determined to be between 1 to 1.2 μg and LD₅₀ dose of 1.2 μg per fish was used as the challenge dose. To confirm the LD₅₀, twenty-one non-vaccinated fish were challenged by intraperitonal injections. Ten fish (50%) died within a week, within this group, 7 fish (33%) died between 2 to 3 days. Further, more than 80% of fish had symptoms of hemorrhage, as observed on fish skin and adipose tissues. All of the fish had lost appetite for a period up to 7 to 10 days after the injections. However, control fish, intraperitonally injected with PBS, had no symptoms of hemorrhage and lost appetite for only one day.

The efficacy of an oral vaccine. Sixteen fish vaccinated by feeding oral vaccine were challenged by intraperitonal injections of 1.2 μg PE toxin. Four vaccinated fish (mortality rate is 25%) died within 5 days. For comparison, among the six fish in the non-vaccinated group (mock vaccination groups with artemia fed with regular E. coli), four fish died within 5 days (mortality rate is 75%). The results show that the oral vaccine containing the recombinant E. coli bioencapsulated by artemia is efficacious. Further, the vaccinated fish also lost appetite, but they recovered 2 to 3 days earlier than those fish survived from PE challenge in the non-vaccinated group.

The optimal time of preparation enriched artemia nauplii. To evaluated the optimal time of prepare enriched artemia, the E. coli (BL21, DE3) that expressed PE protein was fed to artemia nauplii. The artemia nauplii were killed at time 0, 15, 30, 45, 60, 90, and 120 minutes after feeding, and washed five times with PBS to eliminate the superficially attached E. coli. The artemia nauplii samples were suspended in 6 M urea and sonicated. The PE protein extracted from the samples were analyzed by SDS-PAGE and detected by western blotting analysis with rabbit-anti-PE antibody (see above). The results show that the amount of the PE protein in artemia nauplii increases after feeding, reaches a plateau after 30 to 60 minutes, and drops later on. Accordingly, the optimal time for feeding artemia nauplii in this exampled experiment is 45 minutes.

Comparison of recombinant and bacterial native antigens. Six strains of Pseudomonas aeruginosa (ATCC 15693, ATCC 29260, ATCC 33449, ATCC 33354, ATCC 33355 and ATCC 33359) that conferred PE toxin and PE-enriched E. coli recombinant were individually fed to artemia nauplii. PE proteins extracted from seven artemia nauplii samples were analyzed by SDS-PAGE and detected by western blotting analysis with rabbit-anti-PE antibody (see above). Artemia fed with PE-enriched E. coli recombinant contained PE protein that can be easily observed by the PAGE gel analysis but those artemia fed with six strains of Pseudomonas aeruginosa did not show any PE protein detectable in PAGE gel analysis. The results indicated that the feeding of recombinant E. coli with specific antigen did increase the amount of antigen as compared to the prior art that fed artemia with intact bacterial pathogen. See Gomez-Gil et al. (1998) Appl Environ Microbiol 64: 2318-2322. Thus, fish can easily get sufficient PE protein to trigger an immune response after uptake of an oral vaccine containing the recombinant PE-enriched E. coli bioencapsulated by artemia nauplii.

The uptake of the PE protein in fish intestine. Histological observation aided by immuno-assay with anti-PE antibody indicated that in a period of one to three hours after feeding with artemia nauplii, the PE protein was observed in the lumen of the fish intestine, and the uptake reached the maximal amount in two hours after feeding. Data show that particles of the PE protein were found to pass the mucosal and enter the epithelial cell. The particles began to disappear after six hours, and totally disappeared after 12 hours.

The safety of the oral vaccine of this invention was also evaluated. No abnormality was observed in the morphology, appetite, swimming behavior, or death in all fish fed with the oral vaccine.

Example 2

The LD₅₀ of Pseudomonas aeruginosa on Zebra Fish was determined. More specifically, Pseudomonas aeruginosa (ATCC 15692) and E. coli were separately incubated in LB at 26° C. for 18 hours and at 37° C. for 16 hours, respectively. The bacterial cells were harvested by centrifugation and washed twice with phosphate buffered saline (PBS) to remove residual growth medium and cell secretion. They were then resuspended in PBS and adjusted to 2.5×10¹⁰ cells/ml. Six Zebra fish were injected with Pseudomonas aeruginosa at a dose of 10 μl (2.5×10⁸ cells) per fish. Another six Zebra fish were injected with the same dose of E. coli. After the injection, all fish were observed for up to 6 days. Six hours after the injection, those fish injected with Pseudomonas aeruginosa begun to slow down. Meanwhile, hemorrhages appeared on the abdominal surface and two sides of the body. These fish died 12 hours after the injection. They all showed typical symptoms of Pseudomonas symptom infection, e.g., lose of appetite and hemorrhages in the epidermal layer of abdomen tissues. In contrast, those fish injected with E. coli did not show any abnormal swimming behavior or hemorrhages and remained alive during the same period. This observation indicates that Pseudomonas aeruginosa is a pathogen or opportunist pathogen in fish.

Further, 30 zebra fish were divided into five groups, six in each group. They were injected with 10 μl PBS containing 0, 5×10⁶, 5×10⁷, 5×10⁸, and 5×10⁹ Pseudomonas aeruginosa, respectively, and observed for 48 hours. The mortalities of the groups were then determined and summarized in Table 1 below.

TABLE 1 The determination of LD₅₀ of Pseudomonas aeruginosa on Zebra Fish Number of bacteria/infection/fish Mortality in 48 hours 5 × 10⁹ 100%  5 × 10⁸ 83% 5 × 10⁷ 50% 5 × 10⁶ 16% PBS 0

The results indicate that the LD50 is 5×10⁷ Pseudomonas aeruginosa per infection per fish.

Then, the efficacy of the oral vaccine described there in was tested in Pseudomonas-challenged zebra fish. Artemia nauplii fed with PE-enriched E. coli and PE-free E. coli were prepared according to the method described above. Forty-five and twenty zebra fish were fed with the two types of Artemia nauplii twice a day at an eight-hour interval, respectively. One month later, the Zebra fish in these two groups, i.e., Vaccinated and Control Groups, were challenged by injection of 10 μl PBS containing 5×10⁷ live Pseudomonas aeruginosa. After six days, the mortalities of the groups were determined and summarized in Table 2 below.

TABLE 2 Effects of Vaccination on Zebra fish Protection rate Groups Number of Fish Mortality (1- Mortality)% Control Group 20  9/20 (45%) 55% Vaccinated Group 45 10/45 (22%) 78%

As shown in Table 2, Pseudomonas aeruginosa infection caused 45% mortality in the fish not vaccinated. In contrast, only 22% of the vaccinated fish died with 6 days after the infection. These results indicate that the oral vaccine of this invention protects fish from Pseudomonas aeruginosa infection. The relative percentage of protection or Relative Percent Survival (RPS) was calculated to be 51% RPS was determined by the formula: Relative Percent Survival=1−(% mortality in vaccinated group/% mortality in control group (Jarp, A. Tverdal in Statistical aspects of fish vaccination trails, Fish vaccinology, Dev. Biol. Stard. Basel, Karger, 1997 Vol 90, pp. 311-320.)

Example 3

It is known that nervous necrosis virus (NNV), a major fish pathogen, often causes 99% of mortality. Artemia fed with E. coli. containing a vector that encoded a subunit gene of NNV envelope protein was used to introduce the protein in fish and induce immune response, thereby protecting the fish.

The amino acid sequence and nucleotide sequences of this NNV envelope protein are shown below.

(SEQ ID NO: 2) MVRKGEKKLAKPATTKAANPQPRRRANNRRRSNRTDAPVSKASTVTGFGR GTNDVHLSGMSRISQAVLPAGTGTDGYVVVDATIVPDLLPRLGHAARIFQ RYAVETLEFEIQPMCPANTGGGYVAGFLPDPTDNDHTFGALQATRGAVVA KWWESRTVRPQYTRTLLWTSSGKEQRLTSPGRLILLCVGNNTDVVNVSVL CRWSVRLSVPSLETPEETTAPIMTQGSLYNDSLSTNDSKSILLGSTPLDI APDGAVFQLDRLLSIDYSLGTGDVDRAVYWHLKKFAGNAGTPAGWFRWGI WDNFNKTFTDGVAYYSDEQPRQILLPVGTVCTRVDSEN (SEQ ID NO: 1) atggtacgcaaaggtgagaagaaattggcaaaacccgcgaccaccaaggc cgcgaatccgcaaccccgccgacgtgctaacaatcgtcggcgtagtaatc gcactgacgcacctgtgtctaaggcctcgactgtgactggatttggacgt gggaccaatgacgtccatctctcaggtatgtcgagaatctcccaggccgt cctcccagccgggacaggaactgacggatacgttgttgttgacgcaacca tcgtccccgacctcctgccacgactgggacacgctgctagaatcttccag cgatacgctgttgaaacactggagtttgaaattcagccaatgtgccccgc aaacacgggcggtggttacgttgctggcttcctgcctgatccaactgaca acgaccacaccttcggcgcgcttcaagcaactcgtggtgcagtcgttgcc aaatggtgggaaagcagaacagtccgacctcagtacacccgcacgctcct ctggacctcgtcgggaaaggagcagcgtctcacgtcacctggtcggctga tactcctgtgtgtcggcaacaacactgatgtggtcaacgtgtcggtgctg tgtcgctggagtgttcgactgagcgttccatctcttgagacacctgaaga gaccaccgctcccatcatgacacaaggttccctgtacaacgattccctat ccacaaatgactccaagtccatcctcctaggatccacgccactggacatt gcccctgatggagcagtcttccagctggaccgtctgctgtccattgacta cagccttggaactggagatgttgaccgtgctgtttactggcacctcaaga agtttgctggaaatgctggcacacctgcaggctggtttcgctggggcatc tgggacaacttcaacaagacgttcacagatggcgttgcctactactctga tgagcagccccgtcaaatcctgctgcctgttggcactgtctgcaccaggg ttgactcggaaaac.

The experiment was conduced in the same manner described above except that the gene encoding the NNV envelope protein was cloned into a pET24a-BL21 (DE3) expression vector. Recombinant E. coli bacteria transformed with the vector were treated with formalin before being fed to Grouper (Grouper, Epinephelus sp.). The LD50 was determined in the same manner described above except that grouper larva (Epinephelus coiodes) that were 1.5 cm in length (around post hatch 35 days) were used. The LD50 was determined to be 1×10⁵ TCID50.

Forty Grouper (Epinephelus coiodes) fry were divided into two groups (20 in each). The fry in one group were fed twice each day for consecutive two days with Artemia containing E. coli BL21(DE3) (“mock group”). Those in the other group were fed in the same manner with Artemia containing E. coli BL21(DE3) that contained a pET24a-NNV plasmid and expressed the NNV envelope protein (“vaccinated group”). Seven days later, all fish were challenged with NNV at LD50 by injection, and then observed for eight weeks. It was found that, in the vaccinated group, sixteen fish survived (a survival rate of 80%); and only nine fish survived (a survival rate of 45%) in the mock group (See Table 3). The RPS was calculated to be 64%. This result indicates that a single feeding period was sufficient to protect grouper fry from the lethal NNV infection.

TABLE 3 Effects of NNV oral vaccine on NNV-challenged zebra fish Protection rate Test group Cumulative mortality (1- Mortality)% Mock vaccinated group 55% (11/20) 45% Vaccinated group 20% (4/20)  80%

Other Embodiments

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replace by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. For example, an oral vaccine as described above is used for immunization of other domestic animals (e.g., pigs and chickens) or humans. Thus, other embodiments are also within the claims. 

1. A composition, comprising: a bioencapsulating organism which is Artemia, rotifer, alga, or paramecium for use as food for a fish; and a single-cell organism fed to, and bioencapsulated by, the bioencapsulating organism; wherein the single-cell organism is a bacterium or yeast that is transformed and expresses a recombinant antigen, wherein the antigen induces an immune response in the fish against an infectious disease caused by Pseudomonas aeruginosa or nervous necrosis virus, and wherein the antigen is Pseudomonas exotoxin A or nervous necrosis virus envelope protein.
 2. The composition of claim 1, wherein the bioencapsulating organism is Artemia.
 3. The composition of claim 2, wherein the single-cell organism is a bacterium.
 4. The composition of claim 3, wherein the single-cell organism is Escherichia coli or Vibrio.
 5. The composition of claim 1, wherein the recombinant antigen is nervous necrosis virus envelope protein.
 6. The composition of claim 1, wherein the recombinant antigen is Pseudomonas exotoxin A.
 7. The composition of claim 4, wherein the recombinant antigen is Pseudomonas exotoxin A.
 8. The composition of claim 4, wherein the recombinant antigen is nervous necrosis virus envelope protein.
 9. A method for preparing the composition of claim 1, comprising: providing a single-cell organism which is a bacterium or yeast that has been transformed to express a recombinant antigen that induces an immune response in a fish against an infectious disease caused by Pseudomonas aeruginosa or nervous necrosis virus; and feeding the single-cell organism to a bioencapsulating organism which is Artemia, rotifer, alga, or paramecium to obtain a composition containing the bioencapsulating organism that bioencapsulates the single-cell organism wherein the recombinant antigen is Pseudomonas exotoxin A or nervous necrosis virus envelope protein.
 10. The method of claim 9, wherein the bioencapsulating organism is Artemia.
 11. The method of claim 10, wherein the single-cell organism is a bacterium.
 12. The method of claim 11, wherein the single-cell organism is Escherichia coli or Vibrio.
 13. The method of claim 12, wherein the recombinant antigen is nervous necrosis virus envelope protein.
 14. The method of claim 12, wherein the recombinant antigen is Pseudomonas exotoxin A.
 15. A method for orally delivering the composition of claim 1, comprising: feeding a bioencapsulating organism which is Artemia, rotifer, alga, or paramecium to a fish to be vaccinated, wherein the bioencapsulating organism bioencapsulates a single-cell organism which is a bacterium or yeast that has been transformed to express a recombinant antigen, which induces an immune response in the fish against an infectious disease caused by Pseudomonas aeruginosa or nervous necrosis virus, and wherein the antigen is Pseudomonas exotoxin A or nervous necrosis virus envelope protein.
 16. The method of claim 15, wherein the single-cell organism is Escherichia coli or Vibrio and the bioencapsulating organism is Artemia.
 17. The method of claim 16, wherein the recombinant antigen is nervous necrosis virus envelope protein.
 18. The method of claim 16, wherein the recombinant antigen is Pseudomonas exotoxin A. 